东海沿岸潮流峡道海岸剖面发育及其动力机制

PROCESS AND SEDIMENT DYNAMICS OF THE COASTAL PROFILE OF THE TIDAL CHANNEL ADJACENT TO THE EAST CHINA SEA

根据实测资料分析东海沿岸典型潮流峡道海岸剖面的发育条件、沉积地貌相带特征和冲淤演变规律 ,并对海岸剖面形成的动力机制进行探讨。分析研究结果表明 ,峡道海岸剖面的流速切变锋、高含沙量带、沉积速率相关峰值、坡折带和双“S”型剖面 ,是一组直接相关的沉积动力学现象。

The Luotoumen channel and the Maotou channel along the East China Sea coast are typical tidal channels, in which the tidal flow parallel to isobaths dominates dynamically, and the rocky headlands and the seabed trench control the evolution of the coastal profile morphologically. Based on the observed data and the sample analyses, the study shows that there are current shear fronts between tidal flat water and slope water, and between slope water and trench water. The shear fronts lead to sharp decrease in the tidal flow′s carrying sediment capacity, occurrence of the maximum turbidity and the relative peak of accumulation rate near the front. So, the coastal profiles take the shape of double “S”. However, because of the different sediment dynamics, the coastal profiles of the two channels are of different sedimentary facies and processes, especially in the trench. In Luotoumen channel, the tidal flat, with a slower accumulation rate 0.31—0.38cm/a, is covered with the clayey silt of less than 6.0φ medium diameter. Seawards, the accumulation rate increases, and reaches as high as 3.9cm/a and 4.9cm/a at the 15m and 30m water depth, respectively. The surficial sediment becomes finer with 6.0—7.0φ medium diameter. However, in the trench section deeper than 40m, the sediment becomes the mixture of hard mud, shells and gravels, which shows erosion on the trench seabed. In order to maintain a long term equilibrium coastal profile, slumping and sliding of subbottom mud layer may occur. In Maotou channel, the tidal flat, with a accumulation rate 0.5—0.8cm/a,is occupied by the clayey silt of 5.0—7.5φ medium diameter. Seawards, the sediment becomes a bit finer with 7.5—8.0φ, and the accumulation rate adds to the 3.1cm/a at 5m water depth and then decrease to the 2.2cm/a. Contrary to the trench of Luotoumen channel, the trench of Maotou channel has changed from erosion to rapid accretion because the tidal flat or subchannel of the inner bay was largely reclaimed or blocked recently. The seabed is being filled by the high water content, fine grained silty clay, and the accumulation rate reaches over 10cm/a at 45m water depth.